Archive for the ‘fix’ Category

After a several-month hiatus, I finally got the rest of the hardware I needed to finish refitting an old 17″ LCD monitor with LED backlights.

So, without further ado, I give you the LED driver board. It’s a simple mc34063 buck converter, with an extra input from the logic board to control the brightness. It has a mounting hole (more on that folly later) to mount it on one of the existing standoffs.

Many of the components that I used were pulled off the existing driver board: the inductor, freewheel diode, and capacitor. You can look at the schematic and board layout if you like.

Next step, modify the existing driver board so that it cuts off power to the backlights exactly the same way it did before. With a regular CCFL inverter, the logic board controls a PNP (or was it P-channel?) transistor to cut off power to the inverter controller. I hijacked the same circuitry, replacing the existing transistor with a higher-current FU9024N, which not only matched the pinout, but was also scavenged from the original driver.

Now, on to testing. This is why I included a pot–so I could get the perfect 12V maximum current to the LEDs. (it’s hard to see the DMM display, I know)

To get the driver board in the case, I cut out a rectangular section of the old driver board. For perspective, all that bare PCB used to be dedicated to the CCFL backlight inverter. Methinks mine is slightly smaller.

And yes, that’s a bunch of fiberglass dust all over the other parts. At least it’s not conductive, because I’m too lazy to clean it up.

From here, it’s a matter of assembling the whole package.

There were a few speed bumps on this final stretch. I cut the wires for the backlights a bit snug, so it was a bit acrobatic to get it all together. The first time I got it all together, I turned it on to realize that half of my bottom light wasn’t working. With all the abuse it had gone through, one of the pads had lifted. So I had to tear down the whole monitor. And that mounting hole? Well, it didn’t exactly hold the board flat. A little electrical tape solved that problem.